In the early 1960s, the flip chip technology and controlled-collapse-chip connection (C4) technology were introduced by IBM to interconnect ICs to their substrates. A flip chip is a chip that is flipped face down and bonded to the substrate with various interconnection materials and methods such as tape-automated bonding, solder bumps, wire interconnects, isotropic and anisotropic conductive adhesives, and so on. The flip-chip solder preparation process generally composes of UBM (Under Bump Metallurgy), copper and solder bump deposition.
The solder bumping process that IBM invented is expensive and complicated because the solder material is deposited by an evaporation process through a metal mask. Most companies cannot accept the technology due to the cost. The electroplating-based flip chip bumping process can provide fine-pitch solder bumps because of the accurate pattern transfer by the photolithographic process. Although the plating bumping process is cheaper than the metal evaporation process, the facility system is more expensive than a stencil printing process. The thick photoresist process is necessary to form the plating patterns. And the metallization sputtering system is used to deposit the under-bump-metallization layers.
In the stencil printing process, an electroless plating process replaces the sputtering system to form the UBM layer. The stencil printing system can reduce the process cost because the stencil mask replaces the thick photoresist. FIGS. 1a–1c demonstrate the cross-sectional view of the stencil printing method of forming solder bumps. Firstly, the UBM layer is deposited on the pads. The solder paste material is printed on the wafer through the stencil mask with desired patterns. After the stencil mask is removed, the solder bumps form under the defined reflowing temperature profile.
However, the conventional stencil printing process cannot provide the small pitch solder bumps. During the reflowing process, the solder paste is shrunken to form as solder bumps. In order to achieve the designed bump size, the solder paste volume is 40–60% larger than the solder ball. The fine-pitch solder bumps cannot be prepared by the conventional stencil printing process. The UBM layer is critical to the reliability of solder bumps. It is necessary to improve the adhesion of the electroless plated UBM layer.